H_
Matrix Representations
SKIP_ p1 QM_[0;S;not defined;not defined;T;W;A;not defined;defined but not listed;not defined] Recall that the product of an m by n matrix A and an n by p matrix B is defined to be the m by p matrix whose entry in row i and column j is the dot product of row i of matrix A and column j of matrix B. Note that this is defined only if A has the same number of columns as B has rows and that the operation is not commutative. Evaluate the products where

images/ma322c3c1.png

and

images/ma322c3c2.png

AH_[2]

  1. AS_[A;B;C;D;I;Z;R;S;T;U;V;W;not defined;defined but not listed] AB
  2. AS_[A;B;C;D;I;Z;R;S;T;U;V;W;not defined;defined but not listed] BA
  3. AS_[A;B;C;D;I;Z;R;S;T;U;V;W;not defined;defined but not listed] AC
  4. AS_[A;B;C;D;I;Z;R;S;T;U;V;W;not defined;defined but not listed] CA
  5. AS_[A;B;C;D;I;Z;R;S;T;U;V;W;not defined;defined but not listed] AD
  6. AS_[A;B;C;D;I;Z;R;S;T;U;V;W;not defined;defined but not listed] AI
  7. AS_[A;B;C;D;I;Z;R;S;T;U;V;W;not defined;defined but not listed] IA
  8. AS_[A;B;C;D;I;Z;R;S;T;U;V;W;not defined;defined but not listed] AZ
  9. AS_[A;B;C;D;I;Z;R;S;T;U;V;W;not defined;defined but not listed] ZA
SKIP_ p2 QM_[0;6;12;3;6] Let g be the linear map from images/ma322c3c3.png to itself defined by f(1) = 6. AH_[2]
  1. Assuming the standard bases for the domain and codomain, the matrix for g is ( AC_[4] ).
  2. Assuming the standard basis for the codomain and the basis (2) for the domain, the matrix for g is ( AC_[4] ).
  3. Assuming the standard basis for the domain and the basis (2) for the codomain, the matrix for g is ( AC_[4] ).
  4. Assuming the bases for the domain and the codomain are (2), the matrix for g is ( AC_[4] ).
SKIP_ p3 QM_[0;3;1;-5;4;2;-1;-9;13;1;6;-2;5;-5;11;-7;12] Let f be the linear map from images/ma322c3c4.png to images/ma322c3c5.png defined by f((1,0)) = (3,-5) and f((0, 1)) = (1, 4). (As usual, I will often write column vectors as row vectors -- you are expected to interpret them correctly from the context.) AH_[2]
  1. Assuming that one is using the standard bases, the matrix of f is
    AC_[4] AC_[4]
    AC_[4] AC_[4]
  2. Assume that the basis for the codomain is the standard basis but that the basis for the domain is (1, -1), (-1, 2) (in this order). The matrix of f is then
    AC_[4] AC_[4]
    AC_[4] AC_[4]
  3. Assume that the basis for the domain is the standard basis but that the basis for the codomain is (1, -1), (-1, 2) (in this order). The matrix of f is then
    AC_[4] AC_[4]
    AC_[4] AC_[4]
  4. Assume that the bases for the domain and the codomain are (1, -1), (-1, 2) (in this order). The matrix of f is then
    AC_[4] AC_[4]
    AC_[4] AC_[4]
SKIP_ p4 QM_[0;2;-9;-1;13;2;-1;-9;13;2;-1;-9;13;2;-1;-9;13] Suppose the linear map f from images/ma322c3c6.png to images/ma322c3c7.png is defined by the matrix images/ma322c3c8.png where the matrix was defined with respect to the basis images/ma322c3c9.png (in this order) for the codomain and images/ma322c3c10.png (in this order) for the domain. Suppose the linear map g from images/ma322c3c11.png to images/ma322c3c12.png is defined by the matrix images/ma322c3c13.png where the matrix was defined with respect to the basis images/ma322c3c14.png (in this order) for the codomain and images/ma322c3c15.png (in this order) for the domain. Let h = f(g) be the composition of the two linear maps. AH_[2]
  1. One has images/ma322c3c16.png AC_[4] images/ma322c3c17.png AC_[4] images/ma322c3c18.png and images/ma322c3c19.png AC_[4] images/ma322c3c20.png AC_[4] images/ma322c3c21.png .
  2. The matrix of the map h with respect to the basis images/ma322c3c22.png for the codomain and images/ma322c3c23.png for the domain is:
    AC_[4] AC_[4]
    AC_[4] AC_[4]
  3. The product AB is
    AC_[4] AC_[4]
    AC_[4] AC_[4]
  4. The matrix of the map f with respect to the basis images/ma322c3c24.png (in this order) is
    AC_[4] AC_[4]
    AC_[4] AC_[4]
SKIP_ p5 QM_[0;m;n;n;p;true;false;true;true;true] Let images/ma322c3c25.png and images/ma322c3c26.png be linear maps of finite dimensional vector spaces. Let images/ma322c3c27.png , images/ma322c3c28.png , and images/ma322c3c29.png be bases of U, V, and W respectively. Let images/ma322c3c30.png and images/ma322c3c31.png be the matrices of f and g respectively, where each matrix is written using the above bases (in the order specified). Let h = f(g). AH_[2]
  1. The matrix A has AS_[m;n;p;none of these] rows and AS_[m;n;p;none of these] columns. The matrix B has AS_[m;n;p;none of these] rows and AS_[m;n;p;none of these] columns.
  2. AS_[true;false] For each index i = 1, ..., p, one has images/ma322c3c32.png .
  3. AS_[true;false] For each index i = 1, ..., n, one has images/ma322c3c33.png .
  4. AS_[true;false] For each index i = 1, ..., p, one has images/ma322c3c34.png .
  5. AS_[true;false] The entry in row k and column i of the matrix AB is images/ma322c3c35.png .
  6. AS_true;false] The matrix of the composition h = f(g) written with respect to the above bases of W and U is the matrix AB.
SKIP_ p6 QM_[0;true;false] Answer the following questions either true or false; make sure you can prove the assertion or give a counter-example: AH_[2]
  1. AS_[true;false] Because composition of functions is associative, i.e. f(g(h)) = (f(g))(h), we know that multiplication of matrices is associative.
  2. If A is a matrix corresponding to a one-to-one linear map and there is a matrix B with BA equal to an identity matrix, then A is non-singular with inverse B. AS_[true;false]
SKIP_ p7 QM_[0;true;true;true;false;true;true] Let A be any m by n matrix with real entries. Recall that applying an elementary row operation to A is the same as multiplying on the left by a particular non-singular matrix. Let I be the m by m identity matrix (which has 1 along the main diagonal and 0 elsewhere). AH_[2]
  1. AS_[true;false] Swapping rows i and j of A amounts to multiplying it on the left by the matrix obtained by swapping rows i and j and the matrix I.
  2. AS_[true;false] Multiplying row i of A by s amounts to multiplying A on the the left by the matrix obtained by multiplying row i of I by s.
  3. AS_[true;false] Replacing row i of A with the sum of row i and m times row j amounts to multiplying A on the left by the matrix obtained from I by replacing row i of A with the sum of row i and m times row j of I.
  4. AS_[true;false] Suppose that applying row operation images/ma322c3c36.png to a matrix is equivalent to multiplying it on the left by the matrix images/ma322c3c37.png for i = 1, 2, ..., k. Then the result of applying the operations images/ma322c3c38.png (in this order) to the matrix is equivalent to multiplying the matrix on the left by the product images/ma322c3c39.png .
  5. AS_[true;false] Suppose that applying row operation images/ma322c3c40.png to a matrix is equivalent to multiplying it on the left by the matrix images/ma322c3c41.png for i = 1, 2, ..., k. Then the result of applying the operations images/ma322c3c42.png (in this order) to the matrix is equivalent to multiplying the matrix on the left by the product images/ma322c3c43.png .
  6. Now suppose that A is a square m by m matrix. Form the matrix B = (A | I), i.e. B has m rows and 2n columns, the first m columns of B are the same as the corresponding columns of A, and the remaining m columns are those of the m by m identity matrix. Use Gaussian elimination to reduce B to its reduced echelon form C. Then C = DB where D is obtained as in the last part. It is AS_[true;false] that if A is non-singular, the C = (I | D) for some matrix D. It follows from C = DB and B = (A | I) that DA = I and so D is the inverse of A.
SKIP_ p8 QM_[0.000001;4/17;-1/17;5/17;3/17] Use the method suggested by the last problem to find the inverse of the matrix images/ma322c3c44.png . (Enter your answer as exact fractions, e.g. 4/3 rather than a decimal approximation.) AH_[2]
AC_[4] AC_[4]
AC_[4] AC_[4]
SKIP_ p9 QM_[0.00001;5/17;-6/17;2/17;1/17;5/17;-6/17;2/17;1/17] Let f be the linear map of images/ma322c3c45.png to itself defined by the matrix images/ma322c3c46.png where we are using the standard basis. Consider the basis images/ma322c3c47.png . AH_[2]
  1. The matrix of f using the standard basis for the domain and the new basis for the codomain is exactly
    AC_[4] AC_[4]
    AC_[4] AC_[4]
  2. Let images/ma322c3c48.png , then using the results of the last exercise, one can calculate images/ma322c3c49.png to be exactly
    AC_[4] AC_[4]
    AC_[4] AC_[4]
SKIP_ p10 QM_[0;0;4;1;4;2] Consider the matrices corresponding to the following linear maps from the vector space of polynomials of degree at most n to itself. The bases are always the standard one: images/ma322c3c50.png . AH_[2]
  1. For the map f(p(x)) = p(2x), the entry in the 3rd row and 4th column is AC_[4] .
  2. For the map f(p(x)) = p(2x), the entry in the 3rd row and 3th column is AC_[4] .
  3. For the map f(p(x)) = p(x + 2), the entry in the 3rd row and 3th column is AC_[4] .
  4. For the map f(p(x)) = p(x + 2), the entry in the 2nd row and 3th column is AC_[4] .
  5. For the map f(p(x)) = p'(x), the entry in the 2nd row and 3th column is AC_[4] .
SKIP_